Audio reserve alarm mechanism for self-contained breathing apparatus

ABSTRACT

An audio alarm mechanism warns a diver using a self-contained underwater breathing apparatus (scuba) that the air tank pressure has dropped to reserve level. The mechanism includes an oscillator piston having one end which cooperates with an annular seat to gate air flow between the scuba first and second stage regulators. When the air tank pressure is sufficiently high, a stem spaces the piston away from the seat to allow unimpeded air flow to the mouthpiece and to prevent oscillation of the piston. When the tank pressure drops, the stem retracts, permitting oscillation of the piston at an audio frequency. Oscillation is sustained by feedback of the outlet pressure via a passageway to an actuating chamber at the other end of the piston. The resultant acoustic vibrations are transmitted both through the air supply hose and through the water to warn the diver to begin his ascent.

United States Patent [191 Christianson 5] March 6, 1973 1 AUDIO RESERVEALARM 3,244,196 4/1966 Replogle ..137 557 MECHANISM FOR SELECONTAINED3,508,542 4/1970 Browner ..128/142.2

BREATHING APPARATUS Primary Examiner-Louis J. Capozi [75] Inventor:Raymond A. Christianson, In- Anomey 1:1am & Flam glewood, Calif. [73]Assignee: Under Sea Industries, Inc., Comp- [57] ABSTRACT I011, Calif.An audioalarrn mechanism warns a diver using a self- [22] Filed: March1, 1971 contained underwater breathing apparatus (scuba) App]. No.:122,594

US. Cl. ..116/70, 73/419, 128/1422,

that the air tank pressure has dropped to reserve level. The mechanismincludes an oscillator piston having one end which cooperates with anannular seat to gate air flow between the scuba first and second stageregulators. When the air tank pressure is sufficiently high, a stemspaces the piston away from the seat to allow unimpeded air flow to themouthpiece and to prevent oscillation of the piston. When the tankpressure drops, the stem retracts, permitting oscillation of the pistonat an audio frequency. Oscillation is sustained by feedback of theoutlet pressure via a passageway to an actuating chamber at the otherend of the piston. The resultant acoustic vibrations are transmittedboth through the air supply hose and through the water to warn the diverto begin his ascent.

30 Claims, 7 Drawing Figures PATENTEB 61915 3,719,160

SHEET 30F 3 TO MOUTHPIECE 7 g T FROM REGULATOR H 22" FROM REGULATOR-HFROM TANK 14 IN VENTO/Q RAVMO/VD A? CH2 /5 r/fi/vsa/v 5 TO MOUTHPIECE-17 y/4..

A 7 To/eA/Ew- AUDIO RESERVE ALARM MECHANISM FOR SELF-CONTAINED BREATHINGAPPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention Thepresent invention relates to an audio alarm for warning the user of aself-contained breathing apparatus that the pressure in the air supplytank has dropped to reserve level.

2. Description of the Prior Art Exhaustion of the air supply withoutadequate warning is an ever present danger in the use of aself-contained underwater breathing apparatus. Various devices have beensuggested to provide such warning. For example, a widely used reservevalve mechanism utilizes a restriction in the air flow path that can beremoved by operating a lever. The pressure drop caused by therestriction is inconsequential while tank pressure is high, thusadequate pressure is delivered to the unit. However, when tank pressureis low, the pressure drop becomes critical and the diver notes thatextra effort is required to draw sufficient breathable gases. When thiscondition is noted, the diver operates the lever to remove therestriction. Thereupon, sufficient pressure is available for effortlessbreathing and the diver begins his ascent.

More recently, alarms which provide a sonic warning of low tank airpressure have become popular. With such sonic alarms, the diver is notrequired to operate a lever, nor does the diver encounter anyrestriction in air flow. Once triggered, the alarm operates repeatedlywhenever the diver inhales, or continually, whereby the diver isconstantly aware of his peril.

One such sonic reserve alarm, shown in U.S. Pat. No. 2,854,001 toHumblet, incorporates a whistle located in the conduit between the firstand second regulator stages. In other devices, described in U.S. Pat.No. 3,149,752 and No. 3,144,171 to Cousteau-Gagnan, sonic alarms aremounted on the gas tank itself. While this arrangement is functionallysatisfactory, a problem arises in that the diver often exchanges hisempty tank for a full one, thus each tank must be equipped with an alarmmechanism.

Structures shown in U.S. Pat. No. 3,056,378 to Simmonds and No.3,244,196 to Replogle achieve good sound signals by metal strikingmetal, and avoid the disadvantages of a tank-mounted device by locatingthe alarm mechanism between the first and second regulator stages. Boththe Simmonds and Replogle structures are quite complicated. Simmondsutilizes a substantial number of parts to provide a pneumatic relaxationoscillator which commences operation when tank pressure is reduced.Pressure slowly builds up in a capacitor chamber until explosive outflowcauses a striker to engage a rigid object. The outlet recloses and thecycle repeats. The slow charging path is controlled by a snap switchpositioned in accordance with the tank pressure.

The structure of the Replogle patent operates a noise maker on a moresteady state basis as compared with Simmonds. Air expands into a largerdiameter chamber upon opening of a spring-pressed closure. A strikercoupled to the closure causes repeated clanging each time the diverinhales. Replogles noise-maker path is shunted via a pressure-sensingvalve to inhibit the sonic alarm so long as the tank pressure is above acritical value.

Neither Simmonds nor Replogle recognizes that the oscillator principlecan be used in a vastly simpler manner to cause sonic vibrations quiteindependently of mechanical parts striking each other. Accordingly, theprimary object of this invention is to provide an extremely simple audioreserve alarm system of high reliability. A companion object is toprovide an audio alarm mechanism in which maintenance and manufacturingcosts are minimized, all without sacrificing the essential desirablecharacteristics of an audio reserve alarm. These include repeatedoperation at least at every inhalation by the diver, and utilization ofaselfcontained available power source, namely the pressurized breathinggas being supplied to the diver.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided an audio reserve alarm mechanism for use with aself-contained underwater breathing apparatus. The mechanism is providedwith a pneumatic oscillator so arranged as to produce an audio alarm notcaused by impingement of metal upon metal, but instead generated byoscillation of a piston or diaphragm at audio frequency. The acousticalarm signal is transmitted to the diver both via the breathingapparatus hose and through the water itself. Thus, the diver not onlyhears the audio alarm, but also tactilely senses the signal. Thus, evenif the divers hearing is impaired for any reason, he nevertheless willbe warned that the air supply is near exhaustion.

In a preferred embodiment, the inventive alarm mechanism comprises anoscillator piston disposed for reciprocation in a chamber within ahousing. The piston divides the chamber into an inlet region and anactuating region. Air from the breathing apparatus first stage regulatoris supplied to the chamber inlet region, and thence flows between thepiston and an annular seat to an outlet passageway leading to thebreathing apparatus second stage regulator. A feedback path, which maycomprise an opening through the piston or a channel through the housing,communicates between the outlet passageway and the chamber actuatingregion behind the piston. A spring biases the piston toward the annularseat.

An inhibit mechanism spaces the piston away from the seat, permittingunimpeded air flow between the first and second regulator stages whilethe pressure in the air supply tank is sufficiently high. When thepressure in the air supply tank drops to a preselected reserve level,the inhibit mechanism retracts, and the piston is free to oscillate.Upon inhalation by the diver, the reduced pressure in the outletpassageway is communicated via the feedback path to the actuatingchamber. The resultant pressure differential between ends of the pistoninitiates oscillation of the piston to generate the audio alarm.

BRIEF DESCRIPTION OF THE DRAWINGS Detailed description of the inventionwill be made with reference to the accompanying drawings wherein likenumerals designate like parts in the several figures. These drawings,unless described as diagrammatic or unless otherwise indicated, are toscale.

FIG. 1 is a perspective view of a self-contained underwater breathingapparatus incorporating the inventive audio alarm mechanism.

FIG. 2 is an end view, partly cut away and in section, of the housingcontaining the first stage regulator and audio alarm components of theapparatus of FIG. 1, shown attached to a conventional air tank orcylinder.

FIG. 3 is a transverse sectional view of the first stage regulator andaudio alarm mechanism, of FIG. 2, as seen generally along the line 3-3thereof.

FIG. 4 is a fragmentary transverse sectional view of the audio alarmmechanism of FIG. 2, as seen generally along the line 4-4 thereof.

FIG. 5 is a simplified diagrammatic view of an alternative embodiment ofthe inventive audio alarm mechanism.

FIG. 6 is a diagrammatic view of yet another audio alarm mechanismincorporating a diaphragm as the oscillating member.

FIG. 7 is a transverse sectional view of an alarm actuating valve usefulwith the mechanism of FIG. 6, as seen generally along the line 7-7thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detaileddescription is of the best presently contemplated modes of carrying outthe invention. This description is not to be taken in a limiting sense,but is made merely for the purpose of illustrating the generalprinciples of the invention since the scope of the invention best isdefined by the appended claims.

Structural and operational characteristics attributed to forms of theinvention first described also shall be attributed to forms laterdescribed, unless such characteristics obviously are inapplicable orunless specific exception is made.

Referring now to the drawings and particularly to FIGS. 1 and 2 thereof,there is shown a self-contained underwater breathing apparatus (scuba)10 of the type typically used for aquatic diving. Apparatus 10 comprisesa first stage regulator, generally designated 11, included in a housing12 having a yoke 13 adapted for attachment to a tank or cylinder 14 inwhich air is stored under pressure. Air at reduced pressure is suppliedfrom the regulator 11 via a hose 15 to a second stage demand regulator16 incorporated in this instance in a mouthpiece 17. The apparatus 10further includes an audio alarm mechanism 18, also situated withinhousing 12, which provides an audible warning to the diver when the airpressure in tank 14 drops below a reserve level. This audio alarm 18advises the diver to begin his ascent while a sufficient reserve of airstill is available.

As best seen in FIGS. 2, 3 and 4, high pressure air from the tank 14 issupplied via a channel 21 to the first stage regulator 11, which in turnprovides at an outlet channel 22 air at a constant small pressure (e.g.,I40 p.s.i.) over and above that of the ambient medium. This low pressureair is supplied to the mouthpiece 17 via a path including an annularchamber 23, a space 24 between one end 25a of an oscillator piston 25and an annular seat 26, and an outlet passageway 27 to which hose 15 isconnected.

So long as the pressure in the tank 14 is above reserve level, thepiston 25 is held away from the annular seat 26 by a stem 29 (FIG. 4),thereby maintaining the space 24 open. To accomplish this, the rear endflange 28 of a stem 29 is subjected to air at tank pressure supplied viathe channel 21 and a branch channel 31. When the tank 14 pressure isabove reserve level, i.e., above about 350 p.s.i., the air pressureexerted on the stern via flange 28 overcomes the counterforce of a biasspring 32, thereby forcing the stem 32 and the piston 25 to the positionshown in solid lines in FIG. 4.

When the air pressure in the tank 14 drops to reserve level, the forceof the bias spring 32 retracts the stem 32 to the position shown inphantom in FIG. 4. Such stem retraction permits the piston 25 tooscillate, generating an audio alarm which warns the diver that only alimited reserve of air remains in the tank 14.

Oscillation of the piston 25 is initiated when the diver inhales,thereby reducing the pressure in the outlet passageway 27 to below thatin the annular chamber 23. This reduced pressure is communicated via asmall diameter opening 33 through the piston 25 to the actuating region34a of a chamber 34 containing the piston 25. The relatively higherpressure in annular chamber 23 is exerted against the piston end 250,urging the piston 25 away from the seat 26. The resultant air flow fromthe chamber 23 through the space 24 causes the pressure in thepassageway 27 to increase. This increased pressure is fed back via theopening 33 to the actuating chamber 34a, equalizing the pressure on thepiston ends 25a and 25b, and permitting the piston 25 to be urged backtoward the seat 26 by a spring 35. Once more, air flow through the space24 is impeded, and continued inhalation again reduces the pressure inthe passageway 27, the feedback opening 33 and the actuating chamber34a, therebyinitiating the next oscillation cycle. The piston 25continues to oscillate, generating the audio alarm, during each diverinhalation period.

By appropriate selection of the mass of the oscillator piston 25 and thediameter-to-length ratio of the feedback opening 33, the piston 25 willoscillate at a relatively low audio frequency, producing an audio alarmsignal which is communicated via the air in the hose 15 to the diver.Moreover, the oscillation of the piston 25 is coupled, partly via thespring 35, to the housing 12, which housing in turn radiates acousticenergy into the water. The resultant pressure pulses transmitted throughthe water and via hose l5 and mouthpiece 17 can be both heard and feltby the diver. Thus, the diver will be warned of the low tank airpressure level even though his hearing may be impaired.

Referring to FIGS. 3 and 4, note that metal-to-metal contact isprevented by providing the oscillator piston end 25a with an O-ring 37seated in an annular dovetail groove 38 aligned with annular seat 26.The oscillator piston 25 also is provided with a peripheral O-ring 39situated relatively closer to the end 25a than to the other piston end25b, thereby permitting a slight tilting movement of the piston 25. Thisarrangement makes the unit very sensitive, so that the piston 25 readilywill go into oscillation.

For simplicity of construction, the communicating- 35. An O-ring 43prevents leakage to or from chamber- 34. Further, as best illustrated inFIG. 4, the piston 25 may comprise a two-piece assembly, including agenerally cylindrical case 25c and an interior plug 25d threadinglyconnected to the case. A vent hole 44 communicates with the bottom ofthe groove 38 to prevent air from becoming trapped behind the O-ring 37.The passageway 27 may be provided with a conventional threaded fitting45 for connection of the hose 15.

The first stage regulator 11 (FIG. 3) includes a generally cylindricalpiston 52 having a knife edge 53 cooperating with a hard plastic seat 54to form a valve for high pressure air supplied from the tank 14 via thechannel 21 and a chamber 55. Air passing the knife edge 53 iscommunicated via the interior longitudinal opening 52a through thepiston 52 to a chamber 56a and thence to the outlet channel 22. Thevalve separation between the edge 53 and the seat 54, and hence theoutlet pressure, is determined by the force exerted on the flangedpiston end 52b by a bias spring 57 and by the ambient water pressurevented to the spring enclosing chamber 56b via one or more holes 58through the body 12. The spring 57 preferably is selected so that theair pressure at the outlet channel 22 is on the order of 140 p.s.i.

Again for ease of construction, the inlet chamber 55 may be formed byboring inwardly from body' end 12a, the opening being capped by anappropriate threaded plug 61 counterbored to receive the valve member54, and sealed with an O-ring 62. The chamber 56a, 56b, in which theflanged piston end 52b reciprocates may be bored inward from the bodyend 12b, and may be capped with a threaded plug 63. A threaded bore 64extending through plug 63 provides an outlet directly from the firststage regulator 11, bypassing audio alarm mechanism 18. Typically, thisoutlet may be used for connection to pneumatic tools (not shown) or toanother second stage regulator (not shown) for use as a backup regulatorin the event that the primary second stage regulator malfunctions, orfor use by another diver in an emergency. Alternatively, the outlet maybe closed offwith a plug 65 as shown in FIG. 3.

An alternative embodiment 70 of the audio alarm mechanism is shownschematically in FIG. 5. In this embodiment, the various elementsdesignated by primed numbers correspond respectively to the unprimed,but like-numbered components of the above described audio alarmmechanism 18.

Note in FIG. 5 that the oscillator piston 25 is not provided with afeedback opening of the type designated 33 in FIG. 3. Rather, a feedbackpassageway 71 is provided within the housing 12' between the outletpassageway 27' and the piston actuating chamber 34a. The dimensions ofpassageway 71 in part determine the oscillation frequency of the piston25 Of course, such alternative feedback passageway arrangement could beincorporated in the audio alarm mechanism 18 by providing a feedbackopening (not shown) within housing 12.

Still referring to FIG. 5, the stem 29 is coaxial with the piston 25'and extends concentrically through a portion 72 of the L-shapedpassageway 27'. Thus, when the pressure in tank 14 is sufficiently high,the air pressure against the stem end 28 will overcome the force of thebias spring 32' and cause stem 29' to maintain piston 25' away from itsseat. This will permit unimpeded air flow from the output of first stageregulator 11 to the mouthpiece 17. When the air pressure in the tank 14is sufficiently low, the spring 32' causes retraction of the stem 29',and the piston 25' goes into oscillation at an audio frequency. Again,acoustic vibrations are communicated to the diver both through the waterand via hose 15.

In FIG. 6 there is shown an embodiment of the invention utilizing adiaphragm 81 rather than an oscillator piston. When the pressure in tank14 is sufflciently high, air from the first stage regulator 11 flows viaan alarm bypass line 82, an open valve 83, a line 84 and a resonatingchamber 85 to hose 15". When the pressure in the tank 14 drops toreserve level, the valve 83 closes, and the air from regulator 11 flowsto the annular inlet chamber 23".

As the diver inhales, the resultant reduced pressure in the resonatingchamber 85 is communicated back via a feedback path 71" to the actuatingchamber 34a behind the diaphragm 81. As a result of the pressuredifferential between chambers 23" and 34a", diaphragm 81 flexes awayfrom the annular seat 26", permitting air flow from inlet channel 22"past the space between the diaphragm 81 and the seat 26" into theresonating chamber 85. The resultant increased pressure is fed backthrough path 71" to chamber 34a", causing diaphragm 81 to flex backtoward seat 26". Accordingly, diaphragm 81 begins to oscillate atarelatively high audio frequency. Again, the acoustic vibrations arecommunicated through hose 15 and via the body 12" and the water to thediver, who then can begin his ascent while air still remains in tank 14.

A typical embodiment of valve 83 is shown in FIG. 7, and comprises apiston 87 urged by sufficient tank air pressure to the position shown.In this position, air from the inlet line 82 can flow through theannular space 88 surrounding the reduced diameter region 87a of thepiston 87 to the line 84. When the pressure in tank 14 drops to asufficiently low level, the force of a spring 89 overcomes the airpressure exerted via channel 21" on the piston end 87b. As a result, thepiston 87 moves to the right (as seen in FIG. 7), and the piston region870 is interposed between the lines 82 and 84, blocking air flowtherethrough. As a result, the air flow from regulator 11 is diverted toalarm mechanism 80 to initiate oscillation of diaphragm 81, causingproduction of .the audio alarm warning.

Intending to claim all novel, useful and unobvious features shown ordescribed, I make the following claims:

1. An audio alarm mechanism for use with a self-contained breathingapparatus of the type including a supply tank of air, a first stageregulator receiving air at tank pressure and providing air at aregulated pressure, and a conduit for conducting air to a second stagedemand regulator, said mechanism comprising:

a. an oscillator member adapted for motion between a flow communicatingposition in which air from said first stage regulator flows past one endof said oscillator member to said conduit, and a flow impeding positionin which air flow from said first stage regulator to said conduit isimpeded,

b. first means for subjecting said oscillator member one end to air atsaid regulated pressure,

c. feedback means for subjecting the other end of said oscillator memberto the demand air pressure in said conduit, and

d. bias means for urging said oscillator member toward said flowimpeding position, a differential between said demand pressure and saidregulated pressure initiating oscillation of said member at audiofrequency to generate said audio alarm.

2. An audio alarm mechanism according to claim 1 further comprising:

alarm inhibit means for maintaining said oscillator member in said flowcommunicating position when the air pressure in said supply tank isabove a preselected reserve level, and forpermitting oscillation of saidmember when the pressure in saidsupply tank drops below a reserve level.

3. An audio alarm mechanism according to claim 1 wherein said oscillatormember comprises a piston.

4. An audio alarm mechanism according to claim 3 wherein said member isdisposed in a chamber, and wherein said feedback means comprises afeedback opening communicating between said conduit and the actuatingregion of said chamber defined by said member other end.

5. An audio alarm mechanism according to claim 4 wherein the mass ofsaid piston, the force of said bias means and the dimensions of saidfeedback opening cooperate to cause oscillation of said piston at arelatively low audio frequency.

6. An audio alarm mechanism according to claim 1 and enclosed in ahousing containing said chamber, said member comprising a pistondisposed for reciprocation in said chamber, and wherein said bias meanscomprises a spring urging said piston toward one end of said chamber.

7. An audio alarm mechanismaccording to claim 6 wherein said feedbackmeans comprises an opening through said piston.

8. An audio alarm mechanism according to claim 6 wherein said firstmeans for subjecting comprises an annular chamber communicating withsaid first stage regulator output and opening into saidpiston-containing chamber one end.

9. An audio alarm mechanism according to claim 8 further comprising anannular seat at said piston-containing chamber one end, and an outletpassageway central of said seat and communicating between said chamberone end and said conduit, said piston being situated against said seatin said flow impeding position and being spaced from said seat in saidflow communicating position.

10. An audio alarm mechanism according to claim 3 wherein said membercomprises a diaphragm mounted across a chamber and flexurally biasedinto contact with a seat at one end of said chamber, and wherein saidfeedback means comprises a resonating cavity communicating between saidchamber and said conduit, and a feedback passageway between saidresonating cavity and the actuating region of said chamber adjacent toother end of said diaphragm.

11. An audio alarm mechanism according to claim 10 wherein saiddiaphragm oscillates between a How impeding position in which saiddiaphragm one end is in contact with said seat, and a flow communicatingposition spaced from said seat to permit an air flow from said firststage regulator through the space between said diaphragm and said seatto said resonating cavity and thence to said conduit.

12. In a self'contained underwater breathing apparatus of the typeincluding an air tank supplying a first stage regulator, and a conduitconnected to a second stage demand regulator, an audio reserve alarmmechanism comprising:

a. a housing including an interior chamber,

b. an oscillator piston disposed for reciprocation in said chamber,

c. an annular seat at one end of said chamber, I

d. an air inlet chamber surrounding said seat and communicating betweensaid first stage regulator and said chamber,

. an outlet passageway central of said seat and communicating betweensaid chamber and said conduit,

f. a feedback passageway communicating between said outlet passagewayand the other end of said chamber, and g a bias spring urging saidpiston toward said annular seat, h. the pressure differential betweenends of said piston resultant during demand by said second stageregulator initiating oscillation of said piston to produce an audioalarm.

13. An alarm mechanism as defined in further comprising:

inhibit means for maintaining said piston spaced from said seat toprevent oscillation of said piston when the air pressure in said tank isabove a preselected value and for permitting oscillation of said pistonto produce said audio alarm'when the air pressure in said tank dropsbelow said preselected value.

14. An alarm mechanism as defined in claim 13 wherein said inhibit meanscomprises a stem situated within said housing and adapted to i movebetween an extended position maintaining said piston spaced from saidseat and a retracted position permitting oscillation of said piston,

a spring biasing said stem toward said retracted position, and

means for subjecting said stem to the force of air at tank pressure,said force overcoming said spring bias to urge said stem to saidextended position only when said tank pressure is above said preselectedlevel.

15. An alarm mechanism as defined in claim 13 wherein the mass of saidpiston and the dimensions of said feedback passageway are selected sothat said position oscillates at a relatively low audio frequency, theresultant acoustic vibrations being transmitted both via said conduitand through the water to provide both aural and tactile warning that airtank reserve pressure has been reached.

16. An alarm mechanism as defined in claim 12 wherein said feedbackpassageway comprises an opening through said piston.

17. An alarm mechanism asdefined in claim 12 wherein said feedbackpassageway comprises a channel through said housing.

18. An alarm mechanism as defined in claim 12 wherein said first stageregulator also is situated in said housing.

19. An alarm mechanism as defined in claim 12 further comprising anO-ring situated in a groove at one end of said piston facing said seat.

claim 12 20. An alarm mechanism as defined in claim 19 wherein saidpiston is adapted for limited tilting motion within said chamber.

21. An audio alarm for indicating to a diver when his scuba air tankpressure has dropped to reserve level, comprising:

a. a housing having an interior chamber,

b. an oscillator piston reciprocally disposed in said chamber anddividing said chamber into an air inlet region at one end and anactuating chamber 10 region at the other end, one end of said piston andthe corresponding one chamber end cooperating to gate air from the scubafirst stage regulator to an outlet passageway leading to the scubasecond stage regulator,

c. an air feedback path communicating between said outlet passageway andsaid actuating chamber region,

d. means for biasing said piston toward said one chamber end, and

e. means for inhibiting reciprocation of said piston until said air tankpressure drops to reserve level,

f. whereby diver inhalation causes reduced pressure in said actuatingchamber, thereby causing said piston to move toward said actuatingchamber region permitting air to be gated from said first stageregulator to said outlet passageway, said biasing means then returningsaid piston back toward said one chamber end, resulting in oscillationof said piston and generation of acoustic vibrations warning the diverof said reserve level condition.

22. An audio alarm according to claim 21 wherein 23. An audio alarmaccording to claim 22 further comprises an O-ring mounted on said pistonone end and cooperating with an annular seat at said one chamber end togate said air.

24. An audio alarm according to claim 23 wherein said O-ring is disposedin an annular, dovetailed groove vented to said actuating chamber.

25. An audio alarm according to claim 23 further comprising an O-ringaround the periphery of said piston and situated relatively nearer tosaid piston one end than to the other piston end, thereby permittinglimited tilting motion of said piston for sensitive initia-' tion ofoscillation.

26. An audio alarm according to claim 21 wherein said means forinhibiting maintains said piston spaced from said one chamber end,permitting unimpeded air flow from said first stage regulator to saidoutlet passageway, while said tank pressure is above reserve level.

27. An audio alarm mechanism according to claim 1 wherein saidoscillator member comprises a diaphragm.

28. An audio alarm mechanism according to claim 27 wherein said memberis disposed in a chamber, and wherein said feedback means comprises afeedback opening communicating between said conduit and the actuatingregion of said chamber defined by said member other end.

29. An audio alarm mechanism according to claim 6 wherein said feedbackmeans comprises a passageway through said housing from said conduit tothe region of said chamber adjacent said member other end.

30. An audio alarm according to claim 21 wherein said feedback pathcomprises a feedback passageway through said housing.

1. An audio alarm mechanism for use with a self-contained breathingapparatus of the type including a supply tank of air, a first stageregulator receiving air at tank pressure and providing air at aregulated pressure, and a conduit for conducting air to a second stagedemand regulator, said mechanism comprising: a. an oscillator memberadapted for motion between a flow communicating position in which airfrom said first stage regulator flows past one end of said oscillatormember to said conduit, and a flow impeding position in which air flowfrom said first stage regulator to said conduit is impeded, b. firstmeans for subjecting said oscillator member one end to air at saidregulated pressure, c. feedback means for subjecting the other end ofsaid oscillator member to the demand air pressure in said conduit, andd. bias means for urging said oscillator member toward said flowimpeding position, a differential between said demand pressure and saidregulated pressure initiating oscillation of said member at audiofrequency to generate said audio alarm.
 1. An audio alarm mechanism foruse with a self-contained breathing apparatus of the type including asupply tank of air, a first stage regulator receiving air at tankpressure and providing air at a regulated pressure, and a conduit forconducting air to a second stage demand regulator, said mechanismcomprising: a. an oscillator member adapted for motion between a flowcommunicating position in which air from said first stage regulatorflows past one end of said oscillator member to said conduit, and a flowimpeding position in which air flow from said first stage regulator tosaid conduit is impeded, b. first means for subjecting said oscillatormember one end to air at said regulated pressure, c. feedback means forsubjecting the other end of said oscillator member to the demand airpressure in said conduit, and d. bias means for urging said oscillatormember toward said flow impeding position, a differential between saiddemand pressure and said regulated pressure initiating oscillation ofsaid member at audio frequency to generate said audio alarm.
 2. An audioalarm mechanism according to claim 1 further comprising: alarm inhibitmeans for maintaining said oscillator member in said flow communicatingposition when the air pressure in said supply tank is above apreselected reserve level, and for permitting oscillation of said memberwhen the pressure in said supply tank drops below a reserve level.
 3. Anaudio alarm mechanism according to claim 1 wherein said oscillatormember comprises a piston.
 4. An audio alarm mechanism according toclaim 3 wherein said member is disposed in a chamber, and wherein saidfeedback means comprises a feedback opening communicating between saidconduit and the actuating region of said chamber defined by said memberother end.
 5. An audio alarm mechanism according to claim 4 wherein themass of said piston, the force of said bias means and the dimensions ofsaid feedback opening cooperate to cause oscillation of said piston at arelatively low audio frequency.
 6. An audio alarm mechanism according toclaim 1 and enclosed in a housing containing said chamber, said membercomprising a piston disposed for reciprocation in said chamber, andwherein said bias means comprises a spring urging said piston toward oneend of said chamber.
 7. An audio alarm mechanism according to claim 6wherein said feedback means comprises an opening through said piston. 8.An audio alarm mechanism according to claim 6 wherein said first meansfor subjecting comprises an annular chamber communicating with saidfirst stage regulator output and opening into said piston-containingchamber one end.
 9. An audio alarm mechanism according to claim 8further comprising an annular seat at said piston-containing chamber oneend, and an outlet passageway central of said seat and communicatingbetween said chamber one end and said conduit, said piston beingsituated against said seat in said flow impeding position and beingspaced from said seat in said flow communicating position.
 10. An audioalarm mechanism according to claim 3 wherein said member comprises adiaphragm mounted across a chamber and flexurally biased into contactwith a seat at one end of said chamber, and wherein said feedback meanscomprises a resonating cavity communicating between said chamber andsaid conduit, and a feedback passageway between said resonating cavityand the actuating region of said chamber adjacent to other end of saiddiaphragm.
 11. An audio alarm mechanism according to claim 10 whereinsaid diaphragm oscillates between a flow impeding position in which saiddiaphragm one end is in contact with said seat, and a flow communicatingposition spaced from said seat to permit an air flow from said firststage regulator through the space between said diaphragm and said seatto said resonating cavity and thence to said conduit.
 12. In aself-contained underwater breathing apparatus of the type including anair tank supplying a first stage regulator, and a conduit connected to asecond stage demand regulator, an audio reserve alarm mechanismcomprising: a. a housing including an interior chamber, b. an oscillatorpiston disposed for reciprocation in said chamber, c. an annular seat atone end of said chamber, d. an air inlet chamber surrounding said seatand communicating between said first stage regulator and said chamber,e. an outlet passageway central of said seat and communicating betweensaid chamber and said conduit, f. a feedback passageway communicatingbetween said outlet passageway and the other end of said chamber, and g.a bias spring urging said piston toward said annular seat, h. thepressure differential between ends of said piston resultant duringdemand by said second stage regulator initiating oscillation of saidpiston to produce an audio alarm.
 13. An alarm mechanism as defined inclaim 12 further comprising: inhibit means for maintaining said pistonspaced from said seat to prevent oscillation of said piston when the airpressure in said tank is above a preselected value and for permittingoscillation of said piston to produce said audio alarm when the airpressure in said tank drops below said preselected value.
 14. An alarmmechanism as defined in claim 13 wherein said inhibit means comprises astem situated within said housing and adapted to move between anextended position maintaining said piston spaced from said seat and aretracted position permitting oscillation of said piston, a springbiasing said stem toward said retracted position, and means forsubjecting said stem to the force of air at tank pressure, said forceovercoming said spring bias to urge said stem to said extended positiononly when said tank pressure is above said preselected level.
 15. Analarm mechanism as defined in claim 13 wherein the mass of said pistonand the dimensions of said feedback passageway are selected so that saidposition oscillates at a relatively low audio frequency, the resultantacoustic vibrations being transmitted both via said conduit and throughthe water to provide both aural and tactile warning that air tankreserve pressure has been reached.
 16. An alarm mechanism as defined inclaim 12 wherein said feedback passageway comprises an opening throughsaid piston.
 17. An alarm mechanism as defined in claim 12 wherein saidfeedback passageway comprises a channel through said housing.
 18. Analarm mechanism as defined in claim 12 wherein said first stageregulator also is situated in said housing.
 19. An alarm mechanism asdefined in claim 12 further comprising an O-ring situated in a groove atone end of said piston facing said seat.
 20. An alarm mechanism asdefined in claim 19 wherein said piston is adapted for limited tiltingmotion within said chamber.
 21. An audio alarm for indicating to a diverwhen his scuba air tank pressure has dropped to reserve level,comprising: a. a housing having an interIor chamber, b. an oscillatorpiston reciprocally disposed in said chamber and dividing said chamberinto an air inlet region at one end and an actuating chamber region atthe other end, one end of said piston and the corresponding one chamberend cooperating to gate air from the scuba first stage regulator to anoutlet passageway leading to the scuba second stage regulator, c. an airfeedback path communicating between said outlet passageway and saidactuating chamber region, d. means for biasing said piston toward saidone chamber end, and e. means for inhibiting reciprocation of saidpiston until said air tank pressure drops to reserve level, f. wherebydiver inhalation causes reduced pressure in said actuating chamber,thereby causing said piston to move toward said actuating chamber regionpermitting air to be gated from said first stage regulator to saidoutlet passageway, said biasing means then returning said piston backtoward said one chamber end, resulting in oscillation of said piston andgeneration of acoustic vibrations warning the diver of said reservelevel condition.
 22. An audio alarm according to claim 21 wherein saidfeedback path comprises a transverse opening through said piston.
 23. Anaudio alarm according to claim 22 further comprises an O-ring mounted onsaid piston one end and cooperating with an annular seat at said onechamber end to gate said air.
 24. An audio alarm according to claim 23wherein said O-ring is disposed in an annular, dovetailed groove ventedto said actuating chamber.
 25. An audio alarm according to claim 23further comprising an O-ring around the periphery of said piston andsituated relatively nearer to said piston one end than to the otherpiston end, thereby permitting limited tilting motion of said piston forsensitive initiation of oscillation.
 26. An audio alarm according toclaim 21 wherein said means for inhibiting maintains said piston spacedfrom said one chamber end, permitting unimpeded air flow from said firststage regulator to said outlet passageway, while said tank pressure isabove reserve level.
 27. An audio alarm mechanism according to claim 1wherein said oscillator member comprises a diaphragm.
 28. An audio alarmmechanism according to claim 27 wherein said member is disposed in achamber, and wherein said feedback means comprises a feedback openingcommunicating between said conduit and the actuating region of saidchamber defined by said member other end.
 29. An audio alarm mechanismaccording to claim 6 wherein said feedback means comprises a passagewaythrough said housing from said conduit to the region of said chamberadjacent said member other end.